In recent years, there have been technical advancements in information display devices in portable electronic equipment. Display cost, size, legibility, and power consumption are particularly important parameters in display design. The liquid crystal display has proven to yield a good balance of these parameters as evidenced by the widespread use of such displays.
One disadvantage of a liquid crystal display is the fact that it is legible only in the presence of ambient light. To overcome this disadvantage, equipment manufacturers often incorporate a light source located adjacent to the display so as to make the display legible, even on low ambient light, or no ambient light conditions. Light emitting diodes, incandescent lamps, luminescent panels, and other light sources have been successfully employed for this purpose. The use of a light source contradicts the low power parameter of a display design. This is especially important in the design of portable equipment, such as portable radio receivers.
In portable equipment battery life is an important consideration. While the liquid crystal display has been a boon to equipment designers because of its effectiveness and low power consumption, the need for a light source to illuminate such a display is counter productive in terms of battery life. Various schemes have been devised to reduce the power consumption of such light sources, adding a user operated switch to control the illumination of the light source is one example.
Portable radio receivers, including portable scanning radio receivers, often employ displays which indicate the channel or frequency currently being received together with other operational information. The user of such a receiver has a need to view the display from time to time, but normally does not view the display continuously. Therefore, the illumination of the display need not be active continuously. It is especially useful to activate the display light source at the time a new signal is received in a scanning receiver so that the user can identify the broadcast being received.
It has been contemplated to activate the display light source coincidentally with the activation of the squelch signal output from the receiver. As such, the display light source would illuminate whenever the receiver squelch opened. This is not a completely satisfactory solution because sometimes the received signal which caused the squelch to open will remain active for an extended period of time, and cause needless current drain from the battery. In such a situation, the user need only view the display at the beginning of the signal reception. Further, such a scheme does not deal with the situation where there is sufficient ambient light to view the display, in which case, the display light source need not be illuminated at all.